Fiber optic gyro with a source at a first wavelength and a fiber optic loop designed for single mode operation at a wavelength longer than the first wavelength

ABSTRACT

A low cost fiber optic gyro includes a Sagnac interferometer configured in a minimum reciprocal configuration and modified to use a 0.8 micron wavelength laser diode as the interferometer light source and 1.3 micron, single-mode fiber for the sensing coil.

DESCRIPTION

1. Technical Field

This invention relates to fiber optic gyros, and more particularly tooptical interferometer type rotation sensors.

2. Background Art

Measurement of rotation rate is required in applications ranging fromrobotic and ballistic missile control, to aircraft and spacecraftnavigation. Performance accuracy ranges from 0.001 to 0.01 degrees/hourfor Inertial Grade spacecraft/aircraft navigation systems (10⁻³ to 10⁻⁴of earth's 15 degrees/hour rotation rate), through Moderate Gradesensing accuracies of 0.02 to 1.0 degrees/second. Intermediate Gradeperformance is in the 0.1 to 10 degrees/hour range.

Although spacecraft navigation usually relies on spinning wheel gyros,advances in laser technology have allowed dual laser beam gyros ("lasergyros") to be used in high performance applications such as aircraftnavigation systems. The laser gyro offers fast startup, small size,lower cost, and most importantly the absence of moving mechanical parts.An outgrowth of the laser gyro is the fiber optic gyro (or "FOG"), whichis an alternative type of interferometric rotation sensor.

The FOG can be smaller, more rugged, and less costly than the lasergyro, making it ideally suited for lower performance (Moderate andIntermediate Grade) applications in the field of advanced projectiles.Projectile applications for which the FOG is particularly well suitedare roll attitude determination, body rate sensing, and seekerstabilization.

The FOG uses a Sagnac interferometer to measure rotation based on theprinciple that the transit time of an optical signal propagating througha fiber optic loop rotating about an axis perpendicular to its plane,varies with the loop rotation rate. The transit delay for two opticalsignals traversing the loop in opposite directions creates a Sagnacphase differential that is proportional to loop rotation rate: ##EQU1##where: S is the Sagnac phase difference in radians, L is the length ofthe fiber loop, d is the loop diameter, λ is the optical signalwavelength, c is the speed of light, and Ω is the loop rotation rate inradians/sec.

Phase detection sensitivity may be uncreased by modulating both opticalsignals with a sinusoidal phase modulator positioned at one end of theloop. The optical transit time delay causes the modulator to act on thecounter circulating light beams at different times, dithering the phasedifference magnitude and permitting use of sensitive AC processing todetect rotation-induced phase differences.

When counter propagating signals of unit intensity are combinedinterferometrically, the intensity (I) is:

    I=1/2*(1+cos P)                                            (Equation 2)

where P is the total phase difference (Sagnac and phase modulation).

The intensity I versus Sagnac phase difference relationship is a cosinefunction. At zero rotation the phase difference is zero and the signalsinterfere constructively to produce a maximum intensity. Loop rotationcreates a phase differential, causing the signals to destructivelyinterfere and reduce the intensity.

Bessel expansion of the intensity expression at the modulation frequency(f) produces the rotational velocity component:

    F=k* sin (S)                                               (Equation 3)

    where k=2*J.sub.1 *[2A* sin (π*f*T)]. (Equation 4)

and the term 2A* sin (π*f*T) is the dithered phase difference modulationof amplitude (A) and modulation frequency f. The coil transit time is Tand, if (A) is fixed, F is maximized when f=1/2T; the coileigenfrequency.

The analog value of F can be measured directly as an indication ofrotation, or the signal amplitude can be continuously nulled by a closedloop serrodyne modulator which adds an optical phase bias in oppositionto the Sagnac phase difference. This is a repetitive linearly rampedphase modulator positioned at one end of the fiber coil. A peak rampamplitude of 2π radians produces an effectively constant phasedifference bias between the oppositely directed beams. The ramprepetition frequency, which is proportional to the phase bias amplitude,provides a measurable representation of the loop rotation rate.

The rotation sensing accuracy critically depends on the counterpropagating signals travelling identical ("reciprocal") optical paths atzero rotation rate (and zero applied bias). The necessary reciprocitycan be assured by arranging the optic elements in a "minimum reciprocalconfiguration" which requires the optical signals to pass through acommon single-spatial-mode filter and a single-polarization filter whenpropagating from the source to the sensing coil and from the coil to thedetector. This ensures that the counter propagating optical signalsreceived by the detector will travel identical paths, associated with asingle spatial mode and a single polarization, even if multiple spatialmodes and polarizations exist in the optical path due, for example, tofiber birefringence effects and scattering and cross-coupling betweenspatial modes.

In practice, when the filtering is imperfect, the FOG offset errorsassociated with residual polarization and spatial-mode-relatednon-reciprocity (as well as several other types of errors) may bereduced through use of a broadband, short coherence length opticalsource and high birefringence optic fiber in the sensing coil. Thismakes the selected mode counter propagating waves incoherent withcertain cross coupled waves.

The FOG offers the potential for good performance and low cost.Components required for FOG fabrication are readily available atwavelengths near 0.8 and 1.3 microns. The longer wavelength offers theadvantage of: generally lower light loss, easier coupling of components,and greatly reduced photorefractive effects in LiNb03 integrated opticdevices.

The selection of component, however, affects the cost/performancetradeoff. If low cost is a primary objective, it may be difficult tochoose between operation at the 1.3 micron wavelength for whichcommunication grade fiber is readily available at low cost, but the mostappropriate light sources are very expensive, or operation at the 0.8micron wavelength at which suitable inexpensive laser sources arereadily available but the fiber is expensive.

DISCLOSURE OF INVENTION

The object of the present invention is to provide a low cost rotationsensor design for use in moderate and intermediate grade fiber opticgyros.

According to the present invention, a Sagnac interferometer is providedin a known minimum reciprocal configuration, including a singlepolarization filter and a single spatial mode filter located in thecommon light path between the source/detector and the sensing loop, butwhich further includes spatial mode conversion in the sensing loop topermit use of sensing-loop fiber which may be multi-moded at thewavelength of the interferometer light source (e.g. a 1.3 micronsingle-mode fiber sensing coil with a 0.8 micron wavelength laser diodelight source) to retrieve at least a minimum level of optical power fromundesired spatial modes into the desired spatial mode.

In further accord with the present invention, the sensing loop singlemode fiber is non-polarization maintaining fiber, and the interferometerfurther comprises a depolarizer located in the sensing loop to preventsignal fading.

In the prior art the use of multi-mode fiber for the sensing coil fiberis usually considered inappropriate. The conventional multi-mode fibersupports a large number of modes and the high degree of singlemodefiltering then required to ensure reciprocity becomes impracticalbecause it extracts and sends to the detector only a very small fractionof the available light. I have found, however, that the requiredfiltering can be incorporated without unacceptably degrading thedetected light levels if the sensing loop fiber supports only a smallnumber of modes, and if mode-conversion means in the sensing loopensures that a reasonable fraction of the light in any undesired modesis returned to the desired mode for detection.

These and other objects, features, and advantages of the presentinvention will become more apparent in light of the following detaileddescription of a best mode embodiment thereof, as illustrated in theaccompanying Drawing.

BRIEF DESCRIPTION OF DRAWING

The sole Drawing FIGURE is a system block diagram of a rotation sensoraccording to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

As known, the minimum reciprocal configuration Sagnac interferometersyield enhanced FOG performance by minimizing output errors associatedwith undesired polarizations and spatial modes. Prior art highperformance FOG systems have typically required, in addition, the use oflow coherence superluminescent diode (SLD) light sources and highlybirefringent (polarization preserving) sensing loop fiber to furtherreduce these errors. The prices, however, of the SLD and polarizationpreserving fiber are a significant portion of the overall sensor cost.

The present invention comprises a minimum reciprocal configurationSagnac interferometer, modified to permit use of a low cost laser diodeoptical source (e.g., at 0.8 micron wavelength) and low costnon-polarization-preserving sensing coil fiber (e.g.. fiber which issingle-moded at 1.3 microns but may be moderately multi-moded at thesource wavelength). The minimum reciprocal configuration in combinationwith added components described hereinafter, produces a rotation sensorsuitable for use in intermediate grade FOGs but at a significantly lowercost than the prior art devices.

Referring to FIG. 1, an interferometer according to the presentinvention 10, includes an integrated optic chip (IOC) 12, a light source14, light detection circuitry 16, an output tap 18 (which may be adirectional coupler or a beam splitter), a fiber optic sensing coil 20,and control circuitry 22. As described in detail hereinafter, thepresent interferometer further includes a depolarizer 23 and a modeconverter 24 connected between the IOC 12 and the sensing loop 20. Anoptional polarizer 25 (shown in phantom) may be located between thelight source and the IOC.

The light source 14 comprises a multi-mode low coherence (widebandwidth) 0.8 micron wavelength laser diode. It is desirable in aninterferometric FOG to use an optical source with a wide line width (lowcoherence) and high power coupling into an optical fiber.Superluminescent diodes (SLDs) have been used extensively in highperformance FOG systems because they offer a good compromise betweenpower coupling and line width. SLDs, however, are very expensive due tolow volume production. Alternatively, multi-mode laser diodes areavailable with 2-3 nanometer bandwidths (about one-fifth that of SLDsbut sufficient to ensure reasonably low coherence) and with coupledoptical output powers approaching one milliwatt, but at less than onetenth the cost of the SLD. This type of laser diode has been selected asthe best cost to performance trade-off for the interferometer lightsource.

The source provides the light beam on output fiber 26 to the tap 18. Thetap rejects a portion of the light (e.g., 50%), which may be used forother purposes, and transmits the remainder through waveguide 28 to theIOC 12. The waveguide 28 is an optical fiber which is single-moded atthe source wavelength. The IOC, which also operates single-moded at 0.8microns, is formed using a two step proton exchange technique describedin a commonly owned, copending application entitled Single-Polarization,Inteqrated Optical Components for Optical Gyroscopes, S/N 329,121, filedMar. 27, 1989 by Suchoski et al.

The IOC includes a single polarization filter 30 and a single spatialmode filter 32 formed in a waveguide section 33. The polarization filterextinction ratio is on the order of 60 dB. The waveguide 33 is the"common path" for propagating the source optical signal to the sensingcoil 20 and for guiding return propagation of the interference signalfrom the coil to the detection cicuitry 16. The spatial-mode filterensures that only selected spatial mode light enters the sensing coiland only selected mode optical power is coupled back from the loop tothe detector.

The filtered optical signal approaching the sensing loop is presented toa beam splitter/combiner 34, e.g. either a Y-junction or a 3 dBdirectional coupler, which divides the source optical signal into twoequal intensity optical signals presented on waveguide sections 35, 36.In the best mode embodiment a phase ("dither") modulator 37 and aserrodyne modulator 38 (each described hereinafter) are connected to theguide sections 35, 36, respectively. The modulated optical signals atIOC connections 39, 40 are presented through the depolarizer 23 and modeconversion means 24 to the opposite ends 42, 44 of the sensing loop.

After circulating through the sensing loop, light returns toward thesource, being combined at splitter/combiner 34 into an interferencesignal which then proceeds back along the common path guide 33, throughthe mode filter and polarizer, to tap 18 which extracts a portion (e.g.,50%) of the signal and couples it through path 44 to detection circuitry16, the remainder of this signal being guided toward the source andeffectively lost. The path 46 may be an optical fiber which may besingle moded at the source wavelength. The detection circuitry mayinclude a known PIN-diode transimpedance amplifier detection system.

In the best mode embodiment, a closed loop serrodyne modulationtechnique is used to measure the Sagnac phase difference. This makes themeasurement insensitive to intensity variations resulting fromfluctuations of the light source or from multi-mode transmission of thelight through the sensing loop. The serrodyne modulator 38 applies alinear ramped phase modulation to each of the counter circulating lightbeams. If the ramp peak amplitude is 2π radians and the flyback isessentially instantaneous, the serrodyne modulation, acting on the twosignals at different times due to optical delay in the coil, adds aneffectively constant bias to the differential phase. The bias can becontrolled by a servo loop within the control circuitry 22 tocontinuously oppose, and null, the Sagnac phase difference. Theserrodyne frequency then constitutes a gyro output proportional to theloop rotation rate.

The phase modulation of the beams is provided by the dither modulator37, which causes the interference signal amplitude to dither. Thisallows for AC detection of the differential phase. The dither amplitudeis at a maximum when the modulation frequency is equal to theeigenfrequency of the fiber sensing coil. This modulation frequency alsooffers other known advantages in reducing certain types of FOGmeasurement errors.

In the best mode embodiment the sensing coil fiber is a 1.3 micron,non-polarization preserving, single mode fiber. The fiber was selectedbecause of its commercial availability and low cost. The cost per meteris approximately one-fifth that of non-polarization preserving singlemode fiber designed to operate at 0.8 microns, and less than 1/20th thecost per meter of 1.3 micron polarization preserving (highbirefringence) fiber.

Multiple modes (typically two to five) may exist in the 1.3 micron fiberwhen operating with 0.8 micron optical signals. In this case it isconceivable that substantially all of the selected mode optical powermight become converted to an undesired mode during propagation throughthe loop, leaving no selected mode light for return to the detector. Toprevent this it is necessary to functionally incorporate mode converter(or "mode-scrambler") means at one or both ends of the loop. Thisensures that some light from any existing mode will couple into thedesired mode before leaving the loop. The light so coupled will passthrough the selected-mode filter 32 to the detector 16.

The mode conversion feature may be accomplished by control of thesensing coil geometry, i.e. by controlling the coil diameter and thefiber winding technique. The range of acceptable diameter values is notcritical. Small diameter values tend to be associated with high windingstresses and fiber distortions which enhance the spatial mode crosscoupling (scrambling). At the same time, a small diameter increases theattenuation of high order modes and so may ultimately lead to singlemode transmission through the coil, obviating the need for scrambling.However too small a diameter (e.g., less than about 2 cm) may alsounacceptably increase the attenuation for even the desired mode.

Subject to physical packaging limits, a large diameter coil providesgreater FOG sensitivity, However, a large diameter (e.g., greater than 8cm), combined with a smooth winding technique with controlled fibercross-overs, may yield little or no spatial mode selection orconversion. In this case it may be necessary to incorporate a separate,discrete mode scrambling means comprising any one of the known scramblerconfigurations, such as a serpentine series of small random fiber bendsat one end of the fiber coil.

In laboratory experiments using a 16 cm diameter 180 meter longrandom-wound coil of conventional single-mode 1.3 micron communicationsfiber it was found unnecessary to include a separate mode scrambler,suggesting that fiber crossovers in such a coil may provide adequatemode mixing.

The use of non-polarization-preserving single mode fiber in the bestmode embodiment also increases the chance of environmentally sensitivepolarization mode coupling into undesired polarizations. To prevent thepossibility that all light might couple out of the desired polarization(polarization fading), the depolarizing means 23 may be included at oneor both ends of the sensing loop. When a low coherence light source isused, a depolarizer for this purpose may, for example, comprise a shortlength of high birefringence (polarization preserving) fiber connectedwith properly orientated polarization axes relative to the IOC, usingmethods known to those skilled in the art. The anti-fading action of thedepolarizer is analogous to that of the mode scrambler described above.It ensures that some light will always return to the detector in thedesired polarization. However, it also ensures that a similar amount oflight will return toward the detector in the undesired polarization, sothe extinction ratio of the polarizer needed to block this light must behigher than if substantially all returning light were in the desiredpolarization state.

The extinction ratio of the IOC polarization filter 30 is on the orderof 60 dB. It is not now known whether larger extinction coefficients canbe obtained by increasing the length of the IOC waveguide 33, or whetherthere is a performance limit at 60 dB beyond which a single-substrateIOC polarizing filter fails to improve with length. Such a limit mightresult from re-entry of previously rejected light into the primarywaveguide.

Tests have shown, however, that polarization related errors in a Sagnacinterferometer employing a proton exchanged IOC based on a lithiumniobate substrate and having an effective extinction coefficient ofabout 60 dB can be substantially reduced by inserting a supplementalpolarizer just ahead of the input to the IO circuit.Polarization-related errors in the output of our experimental gyro weretypically less than 10 degrees/hour equivalent rotation rate when onlythe IOC polarizer was used. However, when a commercial prism typepolarizer specified to have a 60 dB extinction coefficient was insertedahead of the IO polarizer, the errors decreased to less than 1degree/hour.

Depending on the performance accuracy required for a particularapplication, it may be preferable to include the supplemental polarizer25, shown in series with the IOC 12 in FIG. 1. This configurationeffectively separates the two polarizers and achieves enhancedpolarization extinction. The supplemental polarizer need not be of theprism type. It may be an additional, separate-substrate IOC polarizinqelement, or any other known type of fiber polarizer.

Test results have shown that the present rotation sensor exhibits outputnoise and drift no larger than a few degrees/hour, indicating that gyrosconstructed in accordance with the above-described ideas may be usefulin a variety of applications where cost is important and moderateperformance levels are required.

It should be understood that the source and sensing fiber wavelengthsneed not be limited to 0.8 and 1.3 microns. Any relatively shortwavelength light source may be used in combination with any relativelylong wavelength sensing fiber. For example, the source wavelength mayrange from 750 to 900 nanometers and the design wavelength of the fibermay range from 1200 to 1600 nanometers.

Similarly, although the invention has been shown and described withrespect to a best mode embodiment thereof, it should be understood bythose skilled in the art that the foregoing and various other changes,omissions, and additions in the form and detail thereof, may be madetherein without departing from the spirit and score of this invention.

.[.We claim:.]. .Iadd.What is claimed is: .Iaddend.
 1. Aninterferometric rotation sensor, comprising:optical signal source means,for providing a source optical signal .Iadd.having a primary sourceoptical signal wavelength.Iaddend.; optical fiber sensing loop means,for providing, in .[.the.]. presence of .[.loop.]. rotation.Iadd.thereof.Iaddend., a Sagnac phase difference between two sensingloop optical signals propagating in counter circulating pathstherethrough; .[.integrated optic circuit (IOC) means, having asubstrate with a waveguide array formed thereon, said waveguide arrayincluding.]. a bi-directional common path section responsive to saidsource optical signal, said .Iadd.bi-directional .Iaddend.common path.Iadd.section .Iaddend.having a single polarization mode filter and asingle spatial mode filter formed therein to pass .[.select.]..Iadd.selected .Iaddend.mode optical signals having a .[.desired.]..Iadd.selected .Iaddend.spatial mode and a .[.desired.]. .Iadd.selected.Iaddend.polarization mode, .[.said waveguide means further including.]..Iadd.a .Iaddend.beam splitter/combiner means for splitting said.[.select.]. .Iadd.selected .Iaddend.mode optical signal received fromsaid .Iadd.bi-directional .Iaddend.common path .Iadd.section.Iaddend.into said two sensing loop optical signals for counterpropagation through said sensing loop means, and for combining sensingloop optical signals received from said .Iadd.optical fiber sensing.Iaddend.loop .Iadd.means .Iaddend.into a common interference signal forreturn through said .Iadd.bi-directional .Iaddend.common path.Iadd.section.Iaddend., said interference signal amplitude beingdependent on the magnitude of said Sagnac phase difference; detectormeans, for sensing the amplitude of said interference signal; and meansfor coupling said source optical signal to said .[.IOC means.]..Iadd.bi-directional common path section .Iaddend.and for coupling saidinterference signal from said .[.IOC means.]. .Iadd.bi-directionalcommon path section .Iaddend.to said detector means; as characterizedby: .[.said optical signal source means comprising a laser diode havinga source optical signal wavelength;.]. said .[.sensing loop.]..Iadd.optical .Iaddend.fiber .Iadd.sensing loop means.Iaddend.comprising an optical fiber which is designed for single modeoperation at a wavelength which is longer than said .Iadd.primaryoptical signal .Iaddend.source wavelength, and wherein said .[.sensingloop.]. .Iadd.optical .Iaddend.fiber .Iadd.sensing loop means.Iaddend.embodies a signal mode conversion characteristic therein, toprevent .[.sensing loop.]. cross coupling of all optical power from.[.the select.]. .Iadd.said selected .Iaddend.spatial mode to.[.undesired.]. .Iadd.other .Iaddend.spatial modes .Iadd.in said opticalfiber sensing loop means. .Iaddend.
 2. The rotation sensor of claim 1,wherein said .Iadd.optical fiber .Iaddend.sensing loop .Iadd.means.Iaddend.comprises .[.a.]. .Iadd.an optical .Iaddend.fiber coilconfiguration having a fiber winding geometry which causes said.Iadd.optical fiber .Iaddend.sensing loop .Iadd.means .Iaddend.todisplay inherent mode conversion characteristics.
 3. The rotation sensorof claim 1, wherein said .Iadd.optical fiber sensing loop .[.coilconfiguration.]. .Iadd.means .Iaddend.comprises .Iadd.an optical fibercoil configuration having .Iaddend.a discrete mode conversion elementpositioned at one end .[.of said sensing loop coil.]..Iadd.thereof.Iaddend..
 4. The rotation sensor of claim 3, wherein saiddiscrete mode conversion element comprises a serpentine segment of fiberbends positioned at one end of the sensing loop coil.
 5. The rotationsensor of claim 1, wherein said .Iadd.primary .Iaddend.source.Iadd.optical signal .Iaddend.wavelength is on the order of 0.8 micronsand the .Iadd.optical fiber .Iaddend.sensing loop .Iadd.means.Iaddend.optical fiber is designed to operate as a wavelength on theorder of 1.3 microns.
 6. The rotation sensor of claim 1, furthercomprising depolarization means, at least one, located between thesensing loop coil and said .[.IOC combiner/splitter.]. .Iadd.beamsplitter/combiner means.Iaddend., to prevent sensing loop cross couplingof all optical power from .[.the desired.]. .Iadd.said selected.Iaddend.polarization mode to .[.an undesired.]. .Iadd.anotherpolarization .Iaddend.mode.
 7. The rotation sensor of claim 6, whereinsaid depolarization means comprises a length of single mode fiber havinghigh birefringence.
 8. The rotation sensor of claim 1, furthercomprising:serrodyne modulation means, for applying a linear rampedphase modulation of said sensing loop optical signals, to provide aphase bias in opposition to the Sagnac phase difference; and controlcircuitry, for continuously changing the value of the serrodynemodulation frequency in dependence on said interference signal amplitudeso as to cause the phase bias to continuously null the Sagnac phasedifference magnitude, whereby the nulling value of the serrodynemodulation frequency is proportional to said sensing loop rotation rate.9. The rotation sensor of claim .[.6.]. .Iadd.11.Iaddend., furthercomprising discrete polarizer means, positioned between said means forcoupling and said IOC .Iadd.bi-directional .Iaddend.common path.Iadd.section .Iaddend.waveguide segment, to increase the extinctionratio of said IOC single polarization .Iadd.mode .Iaddend.filter. 10.The rotation sensor of claim 5, wherein said .Iadd.primary.Iaddend.source .Iadd.optical signal .Iaddend.wavelength may range from750 to 900 nanometers and the design wavelength of the sensing loopfiber may range from 1200 to 1600 nanometers. .Iadd.11. The rotationsensor of claim 1 further comprising said bi-directional common pathsection and said beam splitter/combiner means provided in a waveguidearray formed on a substrate of an integrated optics chip (IOC) means..Iaddend. .Iadd.12. The rotation sensor of claim 1 wherein said opticalsignal source means comprises a laser diode. .Iaddend.